KR102228612B1 - Ionization pressure gauge as bias voltage and emission current control and measurement - Google Patents

Abstract

Devices and corresponding methods for operating a hot cathode ionization pressure gauge (HCIG) are provided. The transistor circuit can be configured to flow electron emission current with a low input impedance and control the cathode bias voltage. The emission current and cathode bias voltage can be controlled independently of each other without the servomechanism settling time. HCIGs can be calibrated for leakage current.

Description

Ionization pressure gauge as bias voltage and emission current control and measurement

This application claims the benefit of US Regular Application No. 14/795,729, filed July 9, 2015. The entire teachings of the above application are incorporated herein by reference.

Ionizing vacuum pressure gauges can be used in a wide variety of applications such as semiconductor manufacturing, thin film deposition, high energy physics, ion implantation and spatial simulation. The ionization gauge may include both a cold cathode ionization gauge (CCIG) and a hot cathode ionization gauge (HCIG), and some exemplary HCIG designs include Bayard-Alpert (BA), Schultz-Phelps, and three-pole tube gauges. The sensor of a typical hot cathode ionization vacuum pressure gauge includes a cathode (also called a filament), an anode (also called a grid), and an ion collector electrode. In the case of a BA gauge, the cathode is positioned radially outside of the ionization space (anode volume) defined by the anode. The ion collector electrode is disposed within the anode volume. Electrons travel from the cathode toward and through the anode, and are eventually collected by the anode. However, during the movement of electrons, the electrons collide with the molecules and atoms of the gas that make up the atmosphere where the pressure is to be measured, creating ions. Ions generated inside the anode volume are attracted to the ion collector by the electric field inside the anode. The pressure (P) of a gas in the atmosphere _{can be calculated from the ion and electron currents by the equation P = (1/S)(i i} /i _e ), where S is the scaling factor in units of 1/torr ( Gauge sensitivity) and is a characteristic of a particular gauge geometry, electrical parameters and pressure range; i _i is the ion current and i _e is the electron emission current.

The cathode is heated by a current flow initiated by a voltage source to cause electron emission. The voltage source is controlled by the servo mechanism to maintain the desired electron emission current with a fixed cathode bias voltage of, for example, +30 volts. The voltage difference between the cathode bias voltage and the cathode bias voltage of the anode determines the energy of the emitted electrons as they enter the ionization volume. After all, the energy of the electrons affects the ionization current, so the accuracy of the gauge depends on the precise control of the cathode bias voltage. The magnitude of the electron emission current is determined by the heating power applied within the cathode.

Ionization gauges have several electrical feedthroughs with connecting pins extending through the header housing to provide power to the sensor and receive signals from the sensor (each sensor electrode is connected to the feedthrough electrical connection pin or an electrode connection post to a conductor). Made of) is typically included. Electrical insulators can be provided between the feed-through pin and the header housing and other sensor components to maintain operational safety and signal integrity and prevent electrical current from leaking from the feed-through pin to the header housing that connects to the gauge shell.

Servo control of cathode heating current in hot cathode ionization gauges (HCIGs) can be problematic for several reasons. The cost and complexity of high performance servo circuits can be high. Moreover, when the electron emission current settings are changed, or the pressure in the gauge is changed, the cathode bias voltage is affected as the electron emission current and cathode bias voltage are combined in a typical instrument design (one side affects the other). . After a change in pressure or electron emission current, pressure measurements are typically not available for a servo settling time (dead time) that can last as long as 3 seconds. Dead time refers to a transient condition in which the pressure measurements are therefore not corrected and inaccurate as the filament cathode bias voltage has deviated from the desired nominal value (if instrument calibration is in effect). Thus, servomechanism control leads to sections of gauge inaccuracies or unusability.

Moreover, the effectiveness of electrical feedthrough insulators can be hampered by conductive contamination that may accumulate on the feedthrough insulators of ionization gauges such as HCIGs, which will now be described in detail. Contaminants can form an electrically conductive path between the feedthrough pins (conductors) and the header housing of the HCIG sensor, allowing some of the sensor signal currents to flow across the feedthrough insulators. These leakage currents can cause undesirable effects ranging from inaccurate pressure measurements to complete sensor failure. For example, leakage from cathode electrical feedthroughs can lead to inaccurate electron emission current measurements and inaccurate pressure measurements. Moreover, it is desirable to maintain lower electron emission currents, for example, less than 20 microamps (μA) to prolong the cathode lifetime. However, when the leakage currents become sufficiently large for the electron emission current or other signal currents, it becomes necessary to operate the HCIG cathode with higher electron emission currents to maintain pressure measurement accuracy, which reduces the cathode lifetime. Moreover, the anode feedthrough insulators may be contaminated, particularly during degassing procedures in which the anode structures are heated. In addition to the cathode and anode feedthrough insulators, other feedthrough insulators such as ion collector feedthrough insulators may be contaminated and impede sensor operation.

According to embodiments of the present invention, devices and methods are provided for setting the cathode bias voltage and electron emission current separate from each other, which also eliminate dead times that are characteristic of a conventional servo-based measurement method. Moreover, embodiment devices and methods can be used to eliminate the effects of leakage currents in pressure measurement while the HCIG remains in the HCIG's normal use environment, resulting in more reliable pressure measurements and longer gauge service intervals. Embodiments can provide improved pressure measurement accuracy, the possibility of continuous adjustment of a wide range of electron emission currents, faster electron emission current control without dead time, and faster response to pressure changes, as well as reduced manufacturing cost.

The ionization pressure gauge and corresponding method may comprise a cathode configured to be heated to emit electrons with an electron emitting current. The ionization pressure gauge may include a transistor circuit configured to flow electron emission current with a low input impedance and control the cathode bias voltage. The low input impedance can be substantially zero so that the electron emission current can be sensed in the transistor circuit without affecting the cathode bias voltage. The electron emission current may flow to the current measuring circuit, and the current measuring circuit may include a current sensor. The ionization pressure gauge may include a variable heating power source that variably heats the cathode.

The transistor circuit can control the cathode bias voltage regardless of the magnitude of the electron emission current. For example, the transistor circuit may include a field effect transistor (FET) that controls the gate voltage to independently control the cathode bias voltage while flowing an electron emission current between the source and the drain with zero close input impedance. The cathode bias voltage may be equal to the voltage applied to the gate of the FET plus the offset voltage of the FET, and the gate of the FET may be electrically connected to a variable voltage source to variably control the cathode bias voltage. The electron emission current flowing from the source to the drain can be sensed without affecting the cathode bias voltage. The transistor circuit may be electrically connected to the leakage test current source to enable current flow through the transistor with the cathode electron emission current at zero to provide a current offset for accurate electron emission current sensing. The leakage test current source may include a resistor coupled to the anode bias voltage supply. The leakage current range selection switch may be configured to switch the current range of the leakage test current source according to the level of the leakage current.

The ionization pressure gauge may include a circuit for detecting an offset of a transistor in the transistor circuit, and the circuit for detecting the offset may include a diode electrically connected to the transistor circuit.

The ionization pressure gauge may include a microcontroller capable of calculating the difference between the electron emission current measured with the heated cathode and the electron emission current measured with the unheated cathode, the difference being the calibration of the ionization pressure gauge for leakage current. Can be used. The microcontroller may have control signals that are electrically connected to a cathode heating power supply, a cathode bias voltage control input of the transistor circuit, and a leakage current range selector switch. The microcontroller may include an electrical input that is electrically connected to the current sensor to measure the cathode electron emission current.

A method of operating an ionization pressure gauge and a corresponding device comprises heating a cathode to emit electrons with an electron emitting current, controlling the cathode bias voltage through a transistor circuit, and controlling the cathode bias voltage through the transistor circuit with a low input impedance. It may include the step of flowing an electron emission current. The low input impedance can be substantially zero. The step of flowing the electron emission current may be done in the current measuring circuit, and the current measuring circuit may include a current sensor. Controlling the cathode bias voltage may include applying a variable voltage source to the gate of the FET. The cathode bias voltage can be controlled irrespective of the magnitude of the electron emission current. Heating the cathode may include adjustable heating to emit electrons with a user-selected electron emitting current.

The method may include calibrating the ionization pressure gauge for the leakage current by flowing a leakage test current through the transistor circuit with an electron emission current set to zero. The method may include calibrating the ionization pressure gauge for leakage current by using the difference between the electron emission current measured with the heated cathode and the electron emission current measured with the unheated cathode. The leakage test current can flow through a resistor that is electrically coupled to the anode bias supply and is switched according to the level of the leakage current.

The method may include outputting control signals from the microcontroller to a cathode heating power supply and a cathode bias voltage control input of the transistor circuit. The method may further include outputting a control signal from the microcontroller to a leakage current range selector switch in the transistor circuit. The method may include inputting an electrical signal from a current sensor in the transistor circuit to the microcontroller.

The transistor circuit may include a field effect transistor (FET) that allows an electron emission current to flow. The cathode bias voltage may be equal to the voltage applied to the gate of the FET plus the offset voltage of the FET. The method may include calibrating the transistor circuit by detecting an offset of the transistor in the transistor circuit, and calibrating the transistor circuit may include using a diode. The diode is electrically connected to the transistor in the transistor circuit and can be used to facilitate measurement of the offset of the transistor. The current sensor can be used to measure the electron emission current through the transistor in the transistor circuit.

The method may further include changing the electron emission current from one value to another without dead time in the pressure measurement.

The ionization pressure gauge comprises means for heating the cathode to emit electrons with an electron emitting current, means for controlling the cathode bias voltage through the transistor circuit, and means for flowing an electron emitting current through the transistor circuit with a low input impedance. I can.

What has been described above will become apparent from the following more specific description of exemplary embodiments of the present invention as shown in the accompanying drawings in which like reference characters refer to like parts throughout different drawings. The drawings are not necessarily drawn to scale, and emphasis is instead placed upon illustrating embodiments of the invention.
1A is a schematic diagram showing a conventional Bayard-Alpert (BA) hot cathode ionization gauge (HCIG).
FIG. 1B includes graphs of electron emission current and cathode bias voltage for the cathode in FIG. 1A.
1C is a schematic diagram showing the existing BA HCIG of FIG. 1A with a variable electron emission current source.
2A is a schematic diagram illustrating an embodiment HCIG including a field effect transistor (FET) circuit for flowing an electron emission current.
2B is a flow diagram illustrating an embodiment method of operating an ionization pressure gauge, such as the ionization pressure gauge shown in FIG. 2A.
3A is a schematic diagram showing an alternative embodiment HCIG having a FET transistor circuit, a microcontroller, and a circuitry for leakage current mitigation to flow electron emission current.
3B is a more detailed schematic diagram of the HCIG shown in FIG. 3A.
3C is a flow diagram illustrating an embodiment method of mitigating the effect of leakage current.

A description of exemplary embodiments of the present invention follows.

Hot cathode ionization vacuum pressure gauges (HCIGs) are used in a wide variety of applications such as semiconductor manufacturing, thin film deposition, high energy physics, ion implantation and spatial simulation. Many of these applications require high gauge reliability, low failure rates, and good pressure measurement accuracy over many orders of pressure. Moreover, many of these applications require accurate pressure measurements to be repeated at small time intervals and may be unable to withstand the servomechanism settling times that control HCIG's electron emission current control loop. With these considerations in mind, it is very important to increase the HCIG's ability to report very accurate pressure measurements at small time intervals over a long lifetime without taking control loop correction into account.

1A shows a typical Bayard-Alpert (BA) HCIG. The general principles of operation of such a gauge as described above are described, for example, in US Pat. Nos. 7,295,015 and 7,429,863, which are incorporated herein by reference in their entirety. The cathode 208 is configured to be heated by the current supplied by the cathode heater power supply V _H. The cathode heater power supply V _H is controlled by the servo mechanism 107 by a control signal 109. Cathode 208 is maintained at _{a cathode bias voltage V C} , which may be +30 V, for example. The measurements are valid and calibrated when the voltage sensor 165 monitoring the cathode bias voltage (V _C ) reads the specified working cathode bias voltage (eg, +30 V).

When electrically heated, the cathode 208 emits ^{electrons e-towards the anode 206.} _{This electron emission is limited by the electron emission current i e} , which is an equivalent positive current flow in the direction opposite to the electron flow. 1A, the anode may be configured as a cylindrical wire grid (anode grid) defining an anode volume (ionization volume). The ion collector electrode 217 is disposed within the ionization volume. The anode bias voltage accelerates ^{electrons (e −} ) away from the cathode and towards and through the anode 206. The anode is maintained at the anode bias voltage (V _{A ), which is typically +180 V.} Ultimately, all electrons emitted from the cathode are collected by the anode. During the movement of electrons, powerful electrons collide with gas molecules and atoms where they may be present, creating cations. The cations are then driven to the ion collector electrode 217 by an electric field generated in the anode volume. The electric field can be generated, for example, by an anode that can be maintained at +180 V and an ion collector that can be maintained at, for example, a ground potential. A collector current is then generated in the ion collector, and the pressure of the gas in the ionization volume can be calculated from the ion current. The ion collector 217 measures the ion collector current and is connected to an electric meter (transimpedance amplifier picoammeter) 223 that is normally operated at virtual ground.

The purpose of the servo mechanism 107 is to keep _{the cathode bias voltage V C} at the bottom of the cathode at exactly +30 V. If the voltage V _C drops below +30 V, then the servo mechanism 107 increases the cathode heating power, which increases the electron flow between the cathode 208 and the anode 206, and the voltage at the bottom of the cathode. Pull up. On the other hand, if the voltage V _C rises above +30 V, the servo mechanism 107 reduces the cathode heating power which reduces the electron flow and makes it possible for _{the voltage V C to drop.} At a given cathode bias voltage V _C , the amount of _{electron emission current i e that} will flow in the servomechanism equilibrium state can be selected by the _{switch S e.} Optimal electron emission current depends on gas pressure, desired cathode lifetime, measurement accuracy, etc. The switch S _e is controlled by a command signal 111 from a microcontroller (not shown). In the leftmost switch position, the system _{will be valid and calibrated when the electron emission current (i e} ) = +30 V/10 kΩ = 3 mA. The different electron emission current selections corresponding to the different switch positions are +30 V/100 kΩ = 0.3 mA and +30 V/1 MΩ = 30 μA, respectively.

There are several disadvantages of existing HCIGs such as the existing HCIG shown in FIG. 1A. First, a switch (S _e) has only a limited number of positions. At any given switch position, the electron emission current is still typically far from the optimal tradeoff between measurement accuracy and gauge life. It is desirable to maintain electron emission currents less than, for example, 20 microamps (μA) to prolong the cathode lifetime. However, with the presence of leakage currents, the actual electron emission current may not be perceived, and the HCIG cathode must be operated with a sufficiently high electron emission current to exceed the leakage currents and maintain pressure measurement accuracy. Moreover, a relatively complex and expensive servo mechanism 107 is typically required to minimize dead times and provide the desired accuracy. Moreover, since any real servo implementation has a non-zero settling time and control error, the actual value of the cathode bias voltage (VC) often deviates significantly from +30 V.

FIG. 1B shows the effects of the non-zero settling time of the servo mechanism 107 in FIG. 1A. The upper graph of FIG. 1B shows the electron emission current i _e over time, while the lower graph of FIG. 1B shows the cathode bias voltage V _C over time. As shown in the upper graph, a switch (S _e) is changed and the position at the time (130a), which immediately drops the cathode bias voltage (V _{C). The servo mechanism 107 will ultimately cause the voltage V C} to rise again (by increasing the cathode heating power which increases the electron emission current), but this will cause the time period 132a (dead time) for which pressure measurements are not available. )need. The settling time 132a may be up to 3 seconds, which is an industry standard settling time, for example. This operation is often unacceptable, as some HCIG users require a valid pressure update, for example every 25 ms. As also shown in FIG. 1B, at time 130b, the gas pressure of the HCIG can change rapidly, causing both the electron emission current i _e and the cathode bias voltage V _C to rise or fall temporarily. Over period 132b, the measurements of pressure are likewise invalid. In typical HCIGs as shown in Fig. 1B, the electron emission current and cathode bias voltage (V _C ) are "coupled" or one affects the other.

1C shows an alternative existing approach to electron emission current control. In the schematic diagram shown in Figure 1c, the group of switches (S _e) and a resistor is replaced with a variable current source (115). The current source 115 is controlled by a command 113 from a microcontroller (not shown). This solves the problem of being limited to a few distinct pre-selected electron emission current selections, and thus, the electron emission current can be of any value.

However, the design architecture of Fig. 1C introduces serious problems. The node at the bottom of cathode 208 has an approximately infinite impedance to ground, resulting in a cathode bias voltage (V _C ) that is extremely responsive to cathode power and electron emission current. The servo mechanism 107 is extremely complex, less accurate, and potentially unstable. The servo mechanism 107 can take a very long time to correct. Thus, the advantage of having an adjustable current source 115 comes with significant tradeoffs in circuit stability and reliability.

According to embodiments of the present invention, problems associated with the coupling or dependence between the electron emission current and the cathode bias voltage can be overcome. The transistor circuit can be used to independently control the electron emission current and the cathode bias voltage. Such a transistor circuit can allow electron emission current to flow with very low input impedance, while controlling the cathode bias voltage independent of the electron emission current. The servomechanism 107 with the accompanying settling times shown in FIGS. 1A-1C can be removed to provide continuously valid pressure measurements regardless of gas pressure or electron emission current. Moreover, some embodiments may provide relief of leakage currents, making pressure measurements more accurate over a longer gauge life.

2A is a schematic diagram showing an HCIG having a transistor circuit 220 that controls a cathode bias voltage and flows an electron emission current with a low input impedance. The transistor circuit 220 of HCIG shown in FIG. 2A includes a common gate metal oxide semiconductor FET (MOSFET) 221 and a current that replaces _{the switch S e, the voltage sensor 165 and the servo mechanism 107 of FIG. 1A.} Both sensors 219 are included. Current sensor 219 can be, for example, an ammeter or any current sensing device or circuit. In other embodiments, transistor circuit 220 includes a single MOSFET 221 and current sensor 219 plus additional electrical components.

The command signal 209 from the microcontroller (not shown) variably controls _{the variable cathode heating power supply V H to variably heat the cathode.} The command signal 209 also replaces the control signal 109 from the servo mechanism 107 in FIG. 1A. Accordingly, the microcontroller described in addition to the description of FIG. 3A provides a control signal 209 that is electrically connected _{to the cathode heating power supply V H to control the cathode heating power.} The current sensor 219 measures an electron emission current through the transistor 221. The output from such a sensor fed to the microcontroller can be used to control the electron emission current via signal 209. Unlike the servomechanism control of FIGS. 1A and 1C, the feedback is from the sensed electron emission current to control the electron emission current independent of the cathode bias voltage rather than from the sensed cathode bias voltage.

As used herein, "low input impedance" refers to an impedance that is sufficiently small that changes in the electron emission current do not significantly change the cathode bias voltage. For example, the cathode bias voltage tolerance can be ±1.0 V to provide the desired measurement accuracy, and the maximum expected electron emission current can be 10 μA. In such a case, the transistor circuit can provide the desired benefits if the input impedance is less than approximately 1.0 V/10 [mu]A = 100 [Omega]. The input impedance of the transistor circuit will be considered "substantially zero" if the input impedance of the transistor circuit is less than a previously calculated value, such as the input impedance of a typical FET. For example, substantially zero input impedances of the order of 1000 Ω can be achieved with circuits such as the circuits illustrated herein. Moreover, the transistor circuit shown in Fig. 2A can be further improved by adding circuitry that senses or estimates the FET offset voltage and cancels the FET offset voltage. Such improvements can further reduce the effective input impedance to substantially zero input impedances in the range of approximately 1 to 100 Ω. The input impedance of a transistor circuit can vary widely and can depend on the emission current, the specific selection of transistors in the transistor circuit, circuit complexity, and so on.

In FIG. 2A, the electron emission current i _e flows through the common gate MOSFET amplifier transistor 221, which causes a cathode electron emission current to flow between the emitter reference terminal and the collector reference terminal of the common gate MOSFET amplifier transistor 221. The voltage between the emitter and the base control terminal can have a nominal voltage of approximately 1.5 V. Therefore, applying a fixed voltage to the gate (here, +28.5 V) is a nominal voltage of +30 V at the base of the cathode that does not respond to the amount of _{electron emission current i e flowing from the source to the drain (V C} ) Is calculated. Thus, the electron emission current i _e and the cathode bias voltage V _C are "separated" and independent. That is, the transistor circuit controls the cathode bias voltage regardless of the magnitude of the electron emission current. Furthermore, due to the low input impedance at the source, the electron emission current is not significantly affected by the transistor circuit comprising the current sensor 219.

The HCIG of FIG. 2A does not have any dead times such as the intervals 132a and 132b shown in FIG. 1B. Thus, all pressure measurements to HCIG in Fig. 2A are valid and calibrated at any given time. If a different, more optimal electron emission current is desired for any reason, the microcontroller (not shown in Fig. 2A _{) simply sends a different command 209 to the cathode heating power supply (V H} ) to have a significant effect on the cathode bias voltage. Without it, the electron emission current can be changed. The electron emission current can have any of a continuous range of values depending on the cathode power, and there is no limit to a small set of preselected values as in Fig. 1A. Likewise, the cathode bias voltage can be changed easily and quickly by changing the control voltage to the base of the transistor 221 without affecting the electron emission current.

In other embodiments, the transistor circuit may be configured to control the cathode bias voltage with only certain discrete values. However, it is desirable for a transistor circuit, such as in the transistor circuit of Fig. 2A, to variably control the cathode bias voltage through a continuous range of values limited only by the digital resolution of the microcontroller. While the cathode temperature rises or falls to different values with changes in the electron emission current, the pressure measurements continue to be valid. Thus, Fig. 2A shows a method that can be used to measure pressure with an ionization gauge, even while embodiments of the present invention change the electron emission current from one value to another.

2B is a flow diagram illustrating a method of operating an ionization pressure gauge, such as the gauge shown in FIG. 2A. At 241, the cathode 208 is heated to emit electrons with an _{electron emission current i e.} At 243, the cathode bias voltage V _C is controlled through a transistor circuit. In FIG. 2A, for example, the transistor circuit 220 includes a transistor 221 and a current sensor 219. At 245, the electron emission current i _e is _{controlled through V H} and flows through the transistor circuit with a low input impedance to the current measuring circuit. In FIG. 2A, the current measurement circuit includes a current sensor 219. In other embodiments, the current measurement circuit may include any number of components or devices configured to measure the electron emission current.

The circuit implementation shown in Fig. 2A is a common gate MOSFET amplifier. However, other implementations may provide the best performance for a variety of different design situations. Exemplary alternatives include common gate JFET amplifiers, common base bipolar transistor amplifiers and transimpedance amplifiers. All implementations have in common an input impedance that is substantially zero and a voltage output proportional to the electron emission current. However, bipolar transistor implementations are less desirable as some of the electron emission current flows through the transistor base and is unmetered at the current sensor 219. FET implementations, for example, do not have this drawback and are therefore desirable over bipolar transistor implementations.

FIG. 3A is a schematic diagram showing an HCIG circuit in which the direct bias of the FET 221 of FIG. 2A is replaced by a variable voltage source 331. The variable voltage source 331 has a cathode bias voltage control input to receive the gate control signal 327 from the microcontroller 232. Accordingly, the variable voltage source 331 is controlled by the microcontroller 232 to control the bias voltage of the gate (control terminal) of the transistor 221. A variable voltage source 331 is used in the embodiment of FIG. 3A, but a direct input from a microcontroller or a fixed voltage source may be used in other embodiments. As in Fig. 2A, since the FET has an offset voltage between the source reference terminal and the gate control terminal that does not respond to the electron emission current, the cathode bias voltage can be accurately set by the gate voltage plus the offset. In addition, the electron emission current faces a substantially zero input impedance for the advantages discussed above.

The variable voltage source 331, in combination with an offset circuit 334 comprising a diode 335 to +12 V, enables accurate calibration of the _{FET offset voltage V GS.} Specifically, the cathode bias voltage of the cathode 208 is equal to the voltage applied to the gate of the FET (via the power source 331) plus the offset voltage of the FET 221. Diode 335 is electrically connected to the transistor circuit to facilitate detection of the offset of the FET transistor 221. The diode voltage drop is well defined. With zero cathode heating power and zero actual electron emission current, the gate voltage to the FET 221 can be gradually reduced until the current is sensed at 219. In that regard, the FET source gate offset is the difference between (+12.0 V minus diode voltage) and the gate voltage 331. The offset voltage is relatively constant over a wide range of source (emission) currents. This makes it possible, for example, that the voltage at the bottom of the cathode is set very accurately to +30 V, even if there is no direct measurement of the +30 V node. Avoiding any direct measurement of the +30 V node is a useful feature of this embodiment, as any direct measurement of the voltage of the node will emit some non-zero current, which can lead to electron emission current measurement errors. In the embodiment of FIG. 3A, diode 335 is the only component of offset circuit 334 that detects transistor 221 offset. However, in other embodiments, alternative offset circuitry may include any number of components configured to detect the offset of transistor 221 separately or in combination with each other.

Even without diode 335, pressure measurements using HCIG in FIG. 3A can have an accuracy within approximately 5%, for example. However, with the benefits of using diode 335 to calibrate the FET offset voltage, the theoretical pressure measurement accuracy is within approximately 1%, for example. It should be noted that although the diode approach to correcting the FET offset is very compact and economical, alternative approaches to the correction may be used. Also less desirable, the actual cathode bias voltage can be measured instead of setting the FET gate voltage to the desired cathode bias voltage minus the FET offset voltage. This alternative approach also enables the cathode bias voltage to be very accurate without calibration of the FET. However, in this alternative approach, the electron emission current accuracy can be somewhat reduced due to some electron emission current flowing into the cathode bias voltage measurement circuit.

The microcontroller 232 monitors the current flowing through the current sensor 219 through an electrical input 325 that is electrically output from and connected to the current sensor 219. Specifically, the electron emission current monitor input 325 is used by the microcontroller 232 to read the current sensor 219.

3A also shows how leakage current can be mitigated using embodiments of the present invention. HCIGs typically have one or more electrical feedthroughs that carry signals, for example, between each of the cathode and anode, and outside of the HCIG. For example, the electron emission current i _e is carried by one such feed-through pin. These pins are insulated from the gauge header housing and other paths to ground by feedthrough insulators. However, over time, conductive coatings can form on the feedthrough insulators, which can lead to low impedance paths for leakage current. For example, the coatings can reduce the equivalent resistance from teraohms (TΩ) to megaohms (㏁) and in some cases much less from the feedthrough pins to the header housing of the gauge, and the reduced impedance results in the internal electrodes And may enable leakage currents to develop between the header housing or other paths to ground. Insulators can be coated through a variety of physicochemical processes. Line of sight deposits of material that are sputtered from the inner surfaces of the gauge can cause the development of the conductive coatings. The decomposition of precursor gases through thermal or electron impinging processes can produce by-products that bind to the insulators and may also enable conduction of electrical current in the feedthrough insulators. The conductivity of the deposited coatings may be enhanced by further decomposition of the coatings if the feedthroughs operate at elevated temperatures. Contamination on the cathode electrical feedthrough insulators can be made conductive by this surface decomposition mechanism, as the cathode electrical feedthrough insulators typically operate at a higher temperature than the rest of the electrical feedthroughs, for example. Cathode feedthroughs are typically hotter as the cathode feedthroughs are rigidly connected to the incandescent cathode and often exhibit a maximum level of contamination in the header.

As contamination increases, contamination can accumulate and ultimately cause the gauge to fail (eg, by cathode degradation). Pollution is also due to leakage currents causing inaccuracies, if neglected. Leakage currents limit the minimum actual electron emission current that can be used in HCIGs, thus limiting the upper pressure at which HCIGs can be operated. Leakage currents may limit the underlying pressure at which HCIGs can operate due to the need to measure very few ionic currents at these pressures.

)의 표시 도수를 얻을 수 있도록 상쇄될 수 있다. 도 3c와 함께 후술하는 절차는 도 3a에 도시된 바와 같은 전자 방출 전류 경로에 연결되는 누설 테스트 전류원(337)인 부가 전류 공급기를 이용한다. 도 3a의 실시예는 편리한 전류원으로서 애노드 전원 공급기(333)를 이용한다. 다른 실시예들에서, 전류는 트랜지스터 전류원과 같은 별도의 전원 공급기에 의해, 또는 시스템에 이미 존재하는 상이한 전원 공급기에 의해 제공될 수 있다.3A shows one such leakage current path to redirect some current from the electron emission current path. This current flows through the resistance (R _CL) generated in the feed-through insulator contamination (represented as R _CL) is represented by _CL i. Using the procedure described later in conjunction with FIG. 3B _{, the effects of the leakage current i CL} are determined by the microcontroller 232 using the actual electron emission current (

) Can be offset to obtain the display power. The procedure described later in conjunction with FIG. 3C uses an additional current supply, which is a leakage test current source 337 connected to the electron emission current path as shown in FIG. 3A. The embodiment of FIG. 3A uses an anode power supply 333 as a convenient current source. In other embodiments, the current may be provided by a separate power supply, such as a transistor current source, or by a different power supply already present in the system.

In FIG. 3A, two resistors, a 1 MΩ resistor and a 10 MΩ resistor, are electrically connected in parallel to the anode power supply 333 to form a leakage test current source 337. Thus, the leakage test current source 337 includes a resistor coupled to the anode bias supply. Consequently, the MOSFET transistor 221 is electrically connected to the leakage test current source 337 to enable current flow through the transistor 221 even with an electron emission current set to zero. _{The current depends on the position of the leakage current range selection switch (S s} ) controlled by the microcontroller 232 through the electrically connected leakage current range selector signal 329, as an electron emission current path alone, as a 10 MΩ resistor, or in parallel. It becomes possible to flow through 10 MΩ and 1 MΩ resistors. The switch S _s is further configured to switch the current range of the leakage test current source 337 according to the level of the leakage _{current i CL} , as described later. The switch S _s enables more accurate cancellation of leakage currents through a wider range of leakage resistance. However, in other embodiments, a moderately accurate cancellation through a suitable range of leakage resistance R _CL may be performed with a 10 MΩ resistor or a different resistor alone. The use of the leakage test current source 337 will be described later with reference to Fig. 3C.

3B is a schematic diagram of the embodiment HCIG in FIG. 3A. The microcontroller 232 of FIG. 3A is not shown in FIG. 3B. However, various signals to and from the microcontroller 232 are shown in FIG. 3B. The sections of the schematic diagram of FIG. 3B corresponding to the features in the HCIG of FIG. 3A are labeled with the same reference numbers.

The leakage current source 337 of FIG. 3B shows one 10 MΩ resistor as shown in FIG. 3A. _{The switch S S} of FIG. 3A is not implemented in this schematic diagram. As shown in the upper right of FIG. 3B, the cathode bias voltage for the filament 208 is provided by connection to the FET cathode.

The variable gate voltage controller 331 receives a control signal 327 from the microcontroller 232 shown in FIG. 3A. The output of the operational amplifier (operational amplifier) U1 in the controller 331 drives the base of the transistor Q2. The non-inverting input of U1 is the feedback voltage to ensure proper setting of the FET gate control voltage. The output of operational amplifier U1 is then enhanced to a range that can span 10V to 50V before being applied to the gate of FET 221.

In the current sensor 219, the operational amplifier U2 senses the electron emission current and buffers the input voltage, and the output 325 is connected to the microcontroller 232. Resistor R1 is a switchable current sense resistor used for different current ranges.

The optional diode 335 shown in FIG. 3A is CR1 in FIG. 3B. Although the FET source voltage is nominally 1.5 V higher than the gate voltage, this value can vary due to component tolerances. By adjusting the set gate voltage while reading the current from the current sensor 219 (while there is no actual electron emission current) and gradually decreasing the set gate voltage until the electron emission current starts to flow, the offset is perceived with greater precision. And the cathode bias voltage can be set with greater precision.

3C is a flow diagram illustrating an exemplary procedure that can be used to measure and cancel the effects of leakage current in the HCIG shown in FIG. 3A. At 351, the cathode heating power is set to zero through the cathode heating power control 209. Under these conditions, no emission can occur from the cathode 208. At 353, the anode voltage V _A is set to a normal operating value (eg, +180 V). At 355, the cathode bias voltage V _C is set to a normal operating value (eg, +30 V). At 357, the calibration current flowing through the current sensor 219 (i _calibration ) is measured and recorded by the microcontroller 232. It should be noted that the resistance between the anode and the FET source is R _s. When the switch S _S is open, R _s = 10 MΩ. In this case, all the current (i _calibration ) measured by the current sensor passes through the 10 MΩ resistor of the current source 337. Some portion of the current from the 10 MΩ resistor _{flows through the leakage resistor R CL} , while the rest flows down through the MOSFET 221 and current sensor 219. With this calibration method it is not necessary to recognize the partial currents flowing through these two paths. The leakage current can be calculated as the difference between the current _{through R s} and the current _{detected as i calibration} , or i _CL = [(V _A -V _C )/R _S )]-i _{calibration , and the leakage resistance is R CL} = V _C /i can be calculated by _CL.

)가 마이크로제어기(232)에 의해 측정되고 기록된다. 캐소드 바이어스 전압(V_C)이 교정 전류(i_교정)가 측정되었을 때와 동일하므로, 누설 경로를 통한 전류는 i_CL = 30 V/R_CL로 유지된다. 임의의 실제 전자 방출 전류가

의 측정을 위해 FET(221) 및 전류 센서(219)를 통해 전체적으로 흘러내려갈 것이다. 363에서, 마이크로제어기(232)는

이 R_S를 통한 흐름을 포함하므로, 실 전자 방출 전류

=

-

이라고 판단한다. 따라서, 마이크로제어기(232)는 가열된 캐소드로 측정되는 전자 방출 전류와 가열되지 않은 캐소드로 측정되는 전자 방출 전류 사이의 차를 계산하고, 따라서, 상기 차는 누설 전류에 대하여 이온화 압력 게이지의 교정, 즉 전자 방출 전류(i_e)의 측정들로부터 누설 전류(i_CL)의 영향들을 제거하는 것에 사용된다. 따라서, 실 전자 방출 전류(

)는 제거된 누설 전류의 영향을 갖는다.With continued reference to FIG. 3C, at 359, the cathode heating power supply V _H is turned on to the normal operating value of the cathode heating power supply V _H. Emission from cathode 208 then occurs, and normal operation of HCIG begins. At 361, the current flowing through the current sensor 219 (

) Is measured and recorded by the microcontroller 232. Since the cathode bias voltage (V _C ) is the same as when the calibration current (i _calibration ) was measured, the current through the leakage path is maintained at _{i CL} = 30 V/R _CL. Any actual electron emission current is

It will flow down through the FET 221 and the current sensor 219 for the measurement of. At 363, the microcontroller 232 is

Since this R _{contains flow through S} , the actual electron emission current

=

-

It is judged as. Thus, the microcontroller 232 calculates the difference between the electron emission current measured by the heated cathode and the electron emission current measured by the unheated cathode, and thus, the difference is the calibration of the ionization pressure gauge for the leakage current It is used to remove the effects of the _{leakage current i CL} from measurements of the electron emission current i _e. Therefore, the actual electron emission current (

) Has the effect of the removed leakage current.

)가 이하: P = (1/S)(i_i/

)와 같이 사용되는 것을 제외하면 상술한 압력(P)에 대한 식에 따라 마이크로제어기(232)에 의해 계산된다. 따라서 예를 들어, 각각 도 3a 내지 도 3c의 실시예 장치 및 방법을 사용하여, HCIG가 누설 전류의 영향들에 대해 테스트될 수 있다. 이는 HCIG의 정상적 사용 환경에서 원위치에서 진공 하의 게이지로도 행해질 수 있다. 누설 전류는 누설 전류를 반영하는 i_교정을 측정함으로써 테스트된다. 누설 전류는 그 때 예를 들어, 상술한 바와 같이 마이크로제어기(232) 내에서 누설 전류의 영향들을 감산해 냄으로써 대응될 수 있으며, 따라서 압력 측정 정확성을 증가시킨다.Moreover, the pressure measured by the ionization pressure gauge can be calculated and reported with improved accuracy, as the measured pressure may have the effect of the removed leakage current. At 365, as further shown in FIG. 3C, the ionization current i _i is measured. At 367, the pressure is the actual electron emission current (

) Is less than: P = (1/S)(i _i /

It is calculated by the microcontroller 232 according to the above-described equation for the pressure (P) except that it is used as ). Thus, for example, using the embodiment apparatus and method of FIGS. 3A-3C, respectively, HCIG can be tested for the effects of leakage current. This can also be done with a gauge under vacuum in situ in HCIG's normal use environment. The leakage current is tested by measuring the _{i calibration reflecting the leakage current.} The leakage current can then be countered by subtracting the effects of the leakage current in the microcontroller 232 as described above, for example, thus increasing the pressure measurement accuracy.

, 및 더 작은 오류들이 감산 작동으로 누적될 것이다. 스위치(S_s)는 2개의 상이한 누설 소거 전류 중 하나가 선택되는 것을 가능하게 한다. S_s는 전형적으로 개방될 것이지만, S_s는 예를 들어, 누설 전류(i_CL)가 V_C/10 ㏁을 초과할 때, 폐쇄될 수 있다. 따라서, 스위치(S_S)는 누설 전류의 레벨에 따라 누설 테스트 전류원의 전류 범위를 전환하도록 구성된다.Preferably, the total current through a 10 MΩ resistor or both resistors in parallel is slightly greater than the _{leakage current i CL.} In such case, i _calibration is close to zero,

, And smaller errors will accumulate with the subtraction operation. The switch S _s allows one of two different leakage erasing currents to be selected. S _s will typically be open, but S _s can be closed, for example, when the leakage current i _CL exceeds V _C /10 MΩ. Accordingly, the switch S _S is configured to switch the current range of the leakage test current source according to the level of the leakage current.

It should be understood that accurate leakage current cancellation can be performed in many different ways using circuits similar to the circuit of FIG. 3A in various modifications. For example, a variable and programmable leakage current source may be used in place of the current source 337. Such a programmable leakage current source can be adjusted, for example, until the leakage current reaches a minimum analyzeable current level greater than zero measured at the current sensor. In this case, the current measurements at the current sensor 219 will be the actual electron emission current. Also in some embodiments, the cathode heating power need not be turned off during the measurement _{of i calibration.} For example, the anode voltage V _A may be temporarily turned off so that the electron emission current is zero without the need to cool the cathode. These embodiments have the advantage that the measurements of the i _calibration can be performed very quickly with less disturbance of the operation of the HCIG.

In addition to the calibration for leakage current as described in connection with FIGS. 3A and 3C, there are many alternative devices and methods that can be used to calibrate the HCIG for leakage current. Various alternative devices and methods have been identified by Attorney Document No. 5089.3003-000, filed July 9, 2015, inventors Stephen C. Blouch, Paul C. Arnold, Gerardo A. Brucker, Wesley J. Graba, And US Patent Application No. 14/795,706 entitled “Devices and Methods for Decontamination and Feedthrough Leakage Current Detection in Ionization Gauges” listing Douglas C. Hansen. The teachings of the aforementioned application and any other patents, published applications, and references cited herein are incorporated by reference in their entirety.

Although the present invention has been shown and described in particular with reference to exemplary embodiments of the present invention, various changes in form and detail are not departing from the scope of the invention covered by the appended claims. It will be understood by those skilled in the art that it can be done. For example, a single transistor in a transistor circuit can be replaced with a more complex transistor circuit.

Claims (37)

A cathode configured to be heated to emit electrons with an electron emitting current; And
An ionization pressure gauge comprising a transistor circuit configured to flow the electron emission current with a low input impedance and control a cathode bias voltage of the cathode,
The ionization pressure gauge detects ionic current resulting from the emitted electrons and provides an indication of pressure based on the ionic current.
Ionization pressure gauge. The method of claim 1,
Wherein the low input impedance is less than 100 kΩ. The method according to claim 1 or 2,
Wherein the transistor circuit controls a cathode bias voltage irrespective of the magnitude of the electron emission current. The method according to claim 1 or 2,
The ionization pressure gauge further comprising a variable heating power supply for variably heating the cathode. The method according to claim 1 or 2,
Wherein the transistor circuit comprises a field effect transistor (FET) for flowing the electron emission current through a source of a field effect transistor (FET) and controlling a cathode bias voltage with an applied gate voltage. The method of claim 5,
The cathode bias voltage is equal to the voltage applied to the gate of the FET plus the offset voltage of the FET. The method of claim 5,
The gate of the FET is electrically connected to a variable voltage source to variably control a cathode bias voltage. The method according to claim 1 or 2,
And a circuit for detecting an offset of a transistor in the transistor circuit. The method of claim 8,
Wherein the circuit for detecting the offset of the transistor comprises a diode electrically connected to the transistor circuit. The method according to claim 1 or 2,
Wherein the transistor circuit comprises a current sensor to measure the electron emission current through a transistor in the transistor circuit. As a method of operating the ionization pressure gauge:
Heating the cathode to emit electrons with an electron emitting current;
Controlling a cathode bias voltage of the cathode through a transistor circuit;
Flowing the electron emission current through the transistor circuit with a low input impedance; And
Detecting ionic current resulting from the emitted electrons and providing an indication of pressure based on the ionic current.
Containing, the method. The method of claim 11,
Wherein the low input impedance is less than 100 kΩ. The method of claim 11 or 12,
Wherein the cathode bias voltage is controlled irrespective of the magnitude of the electron emission current. The method of claim 11 or 12,
Heating the cathode comprises variably heating to emit the electrons with a variable electron emission current. The method of claim 11 or 12,
Wherein the transistor circuit comprises a field effect transistor (FET) for flowing the electron emission current through a source of a field effect transistor (FET) and controlling a cathode bias voltage with an applied gate voltage. The method of claim 15,
Wherein the cathode bias voltage is equal to the voltage applied to the gate of the FET plus the offset voltage of the FET. The method of claim 15,
Wherein controlling the cathode bias voltage comprises applying a variable voltage source to the gate of the FET. The method of claim 11 or 12,
And calibrating the transistor circuit by detecting an offset of the transistor in the transistor circuit. The method of claim 18,
Wherein detecting the offset of the transistor comprises using a diode. The method of claim 11 or 12,
And using a current sensor to measure the electron emission current through a transistor in the transistor circuit. The method of claim 11 or 12,
And changing the electron emission current from one value to another without dead time in pressure measurement. Means for heating the cathode to emit electrons with an electron emitting current;
Means for controlling a cathode bias voltage of the cathode via a transistor circuit;
Means for flowing the electron emission current through the transistor circuit with a low input impedance; And
Means for detecting ionic current resulting from the emitted electrons and providing an indication of pressure based on the ionic current
Containing, ionization pressure gauge. The method of claim 1,
Wherein the ionization pressure gauge detects ion current resulting from the emitted electrons and provides an indication of pressure based on the ion current. The method of claim 1,
Wherein the low input impedance is less than the ratio of the cathode bias voltage tolerance to the maximum expected electron emission current. The method of claim 24,
The cathode bias voltage tolerance is ± 1.0 V, the ionization pressure gauge. The method of claim 2,
Wherein the low input impedance is less than 1000 Ω. The method of claim 2,
Wherein the low input impedance is less than 100 Ω. The method of claim 11,
The method further comprising detecting ionic current resulting from the emitted electrons and providing an indication of pressure based on the ionic current. The method of claim 11,
Wherein the low input impedance is less than a ratio of a cathode bias voltage tolerance to a maximum expected electron emission current. The method of claim 29,
The cathode bias voltage tolerance is ± 1.0 V. The method of claim 12,
Wherein the low input impedance is less than 1000 Ω. The method of claim 25,
Wherein the low input impedance is less than 100 Ω. The method of claim 22,
And means for detecting ionic current resulting from the emitted electrons and means for providing an indication of pressure based on the ionic current. The method of claim 22,
Wherein the low input impedance is less than the ratio of the cathode bias voltage tolerance to the maximum expected electron emission current. The method of claim 22,
Wherein the low input impedance is less than 100 kΩ. The method of claim 35,
Wherein the low input impedance is less than 1000 Ω. The method of claim 36,
Wherein the low input impedance is less than 100 Ω.

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